SOI 1980 MiscRep80-4€¦ · are crossed by large river systems - the Beaver (north of Latitu:le 54...

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caments on the Mannville (~ Cretaceous) Aquifer in Saskatchewan

by J. E. Cllris topher

Introduction

'lhe formation waters of the rower cretaceous Mannville Fo:rnation in Saskatchewan are in a hydrodynamic state that largely reflects the major structural elements of the fonnation, as well as the existence of a regional recharge area in the central M::mtana uplift and a discharge region in central western Manitoba. Large scale distortions of the regional potentiaretric surface reflect (1) local cells of high pressure water influxing through fractures fran the underlying Paleozoic lime­stones and (2) regional saddles or flattened cones of depression at locales of discharge into the present-day ground and surface drainage system.

Mannville Tectonic Elem:mts

(Ref. Christopher , J.E., 1980)

Just as the surface topography reflects many of the regional tectonic elements , so also do the paleo-topographic surfaces of the Early Cretaceous. The paleo-topographic surface underlying the Cantuar Foz:nation of the Mannville Group can be reconstructed by drawing an isopach map of its basal tne!Tber, the Dina (Fig. 1). This sandy unit partially blankets the old land surface. Terrain rising above the Dina cover is designated uplands; the fonn of the lowlands is indicated by the isopachs of the Dina. '!his surface is divided into five main regions representing .iooependent structural blocks (Fig. 1).

The southwestern uplands (i.e. swift Current, K:i.ndersley and unity) correspond to the eastern extension of the Sweetgrass Arch of Alberta, and are daninated by the SWift Current platfonn (Fig . 2); subsidiary elemmts of which are the Shaunavon I!0110Cline to the west and the Kindersley block or North Battle­ford Arch (Kent, 1968, Fig. 27) to the northwest.

The Swift Current platform, which is termina­ted by the northwesterly- trending Shaunavon­El.ro,,, troughs, has T!OVed up and ~ through­out the ~sozoic against these linears (Christopher, 1974 , p. 105).

The Shaunavon m::mocline and the Kindersley block appear to have T!OVed along the northerly and easterly trending linears depicted. 'lhese

linears which appear to be subsets of the daninant northeasterl y and northwesterly conjugate set, apparently also controlled the location of the northerly trending Jurassic and Cretaceous oil fields fran Rapdan to Success , and the easterly- trending oilfields of the Kindersley region .

The Middle Devonian 180-m thick Prairie Evaporite has had a widespread and continoous :influence on the Mannville basin. 'Ihe halite forms two l:xxlies joined by a neck north of the Elbow trough (Fig. 2). The eastern l:xx1y fonns a rectanguloid mass trending northwest and bounded to the south by the El.bo,,, ... :t'E.yburn linear, confonning areally to the Lower Cretaceous pre-Pense Punnichy Arch. The ardl apparently resulted by downwarp of the Williston Bas.in to the south and the Alberta Basin to the northwest, and flexing of the post-Prairie Evaporite beds, including the Mannville Cantuar Fonnation, over the halite body . The eastern body also conforms areally to the pre-Dina Govan lowland .

The western body of salt underlies the Shaunavon T!Onocline and tre K.iooersley, Unity and Lloydminster blocks, but is largely absent fran under the pre-Dina Maadow Lake lowlands.

This absence is mainly attributable to salt solution and retreat early but mainly after Mannville tiJres. The absence of salt from under the &.,;ift current platform reflects long term solution of depositionally thinner beds ( 75 to 105 rn) during the Paleozoic, but especially during uplift of the platform in the early Mannville (Aptian), and later in the Cretaceous .

The Present Structural Surface

The sub-Dina structural surface (Fig. 3) out­crops (subcrops) in the north and slopes southward fran an el evation of 348 m in the north to minus 612 min the southeast where it plunges into the Williston Basin. North­easterly and northwesterly trending linears criss-cross the surface. The !'!Ost irrl)Ortant of these are the Elbow-Weyburn along the minus 276 m contour east of longitude 107 degrees , and to the north of Latittrle 52 degrees a similar linear , offset 80 km to the northeast, fronting the Unity-Lloydminster terraces. The Kindersley block, furrowed by the easterly trough associated with the pre-Dina drainage

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Fig. l - Elements of Sub-Cantuar Paleotopography and Dina Isopach map , Saska t chewan.

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Fig. 2 - Structural-topographic elements of the pre-Dina and pre-Pense eros ional surfaces, st ructural linear s and the Prairie Evaporite.

Fig . 3 -

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Struc t ure co t he Pre- in ntours of D. a e r os i on ,

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Saskatchewan.

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Fig. 4 - Formation water potenti ometric map of the Mannvi lle Group , Saskatchewan. Datum sealevel (val ues calculated from shut-i n bottom hole pressures converted to me ters of f resh water rise).

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is prominent, as is the Punnichy Arch.

The structural surface records not only fea­tures of the pre-Dina topography but also the mxlifications since that tine. Thus ll'OSt of the pre-Dina troughs and linears are ll'Odified by extensions and reversals t ypical of salt solution phenarena. The nost striking of the differences however, is the tectonic reversal of the regional gradient; for the Swift CUrrent platfonn which in early Mannville ti.Ires corrmanded the heights of the region OCM lies near the base of the regional dip . Its elevation is 620 m bela.; the Peter Pond Lake area at Latitude 56 degrees near the J:.:order with Alberta. This, when contrasted with the pre-Dina e levation of 590 rn al:.:ove the said reference area , indicates that the Swift Current platfonn has been lo.vered 1210 rn. !lbst of this noverrent reflects the Lararn:ide orogeny and its negative effect on the Williston Basin.

The Potentianetric surfaoe

A regional potentionetric map for water in the Mannville formation (Fig . 4), as ~ted fran the drill stem tests , reveals a potentio­metric surface which declines from atove 800 rn in the southwest to a regional l o.v below 300 m in the northeast. !lbreover, the regional trends are northwesterly and northeasterl y and are delineated by sharpened gradients and broader flats. M::>re specifically the poten­tiorretric surface map shows that:

1. values exceeding 800 m occur as closed contours or potentiaretric cells (Christopher, 1974 , ch. v) in the south­west near Maple Creek and in the south near Ogema , east of the Third Meridian (106 degrees) ;

2. these potentiooetri c cells are nuclei of regional ridges (delineated by the 700 m oontour) oriented toward the northwest along the eastern and western flanks of the Swift current pl atform i.e., along the major l inears of the southwest;

3. potenti anetric values across the Swift CUrrent pl atform lie within a spread of 100 rn betv.ieen the 700 and 600 m contours , thereby indi cating a region of quasi­stability within an overall surface declining ta.,;ard the northwest ;

4. in the regional decl ine toward the north­east, e l evations fall belc,..., those of the outcrops in the Wapawekka ( 389 rn) , Pasquia and Porcupine (380 rn} Hills;

5. in the northwest (north of Latitude 54

degrees and west of Longitude 107 degrees) , the potenticnetric elevations coincide with those of the large lakes such as the Peter Pond-Churchill-Ile a la Crosse system (420 rn) and Dore (458 m);

6. there are four regional lc,...,s or saddles in the west, the largest of which is in the Lloydminster heavy oil district between Latib.rles 54° and 52° where values, as contoure:1 on a 50-ill interval, form a depression (400-450 rn) encorrpassing rrost of the heavy oil pools. Three of the lows are crossed by large river systems - the Beaver (north of Latitu:le 54 degrees), the North Saskatchewan at Lloyaninster and the South Saskatchewan at Latitude 51 degrees (700 m close:1 contour) ; and

7. in the Kindersley region the pressure r idge (700 m) fran the south flanks the southern edge of the oilfiel d district at Latitu3e 51 degrees .

In the Swift CUrrent region, formation waters in the Jurassic and Mannville san:lstones are pressurized by cross-formational f low f ran the under lying Paleozoic lirres tones (Christopher, op. cit.) • This fla,,, occurs through vertical fractures that act as conduits or cel ls frcm which the water spread down-dip and eastward to 1) trap the Jurassic Shaunavon oil against permeability wedgeouts of sandstone into shale and 2) trap the Upper Jurassic Roseray and Cretaceous Success oils at the up-dip uncon­fonnity-truncation of these sands against the l ess perrreable cantuar infill. Here the reservoirs occur in rresas, buttes and praron­tories of the pre-Dina topography. The rreteoric source of this formation water has been sh:Mn to be Paleoz.oic lirrestones in the central Montana uplif t (op. cit.). The a lignrrent of the Jura-cretaceous oilfields against the western pressure ridge i s a result of the so..1thwesterl y dip and buoyancy for oes acting to l ift the oil up-dip.

I n the Lloydrninster region wher e the Mannville lies 500 to 600 m below grotmd, the formation flo.v fran the south is countered by rreteoric water intake fran a broad belt to the north of the North Saskatchewan River . The coinci­dence of the east-west potentiaretri c trough with the course of the river indicates that the river is tapping the Mannville Fornation through its associated glacial arrl pre- gl acial vall ey gravels . This i s nore than merely suggested by the fact that the potenti~tric l evels are within 100 m of the average ground height and well within reach of the pre­gl acial valley bottan. The distribution of the heavy oils throughout this potenti~tric trough indi cates that oil migrating fran the south and southwest under the hydrodynamic

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drive was halted across the shallow dip of the Lloydminster shelf by the inflo,., fran the north. This being a fre sh water influx , it also biodegraded the oil as docunented by Evans et al. , (1971). Likewise to the north and south, the Beaver and South Saskatchewan River systems intercept the .Mannville JX)ten­tionetric surface. Influx to these rivers \\Ould be through fractures and faults in the overlying Cr etaceous shal es .

The Mannvill e heavy oils of the Kindersley district are hardly distinguishable f ran like oils in the Coleville sandstones of the Mississippian Bakken Formation (thus both are plotted on the !Mp) • These reservoirs are located at and in the vicinity of the up-dip truncation of the Bakken beds on the sub-Mannville unconformity . Oil leaking fran these beds is also trapped in the "Detrital" (Success) and other Mannville sandstones.

In southeastern Saskatchewan a pressure ridge trends northeast fran WeybtllTl to Broadview where it ends in a JX)tentiorretric cell . This pressure ridge corresJX)nds areally to the .Rocanville-Torquay trend (Olristopher, 1961 , Fig. 29) , a structural belt marked by salt sol ution sinks , fractures and oil sho..Js . The Mannville wapella oil pool lies inrnediately to the southeast of the 700 m cell as sha.vn. The inability to offset this pool by subsequent drilling , the presence of the cell , and the location of the pool on a nose within a U- shaped trough suggestive of a salt solution feature , all indicate that the pool is a vertical off- shcot of the Mississippian oil fields. The driving rrechanism appears to be the potentiaretric cell .

An arguirent has been made for the migration of oil into the r eservoirs of the southwestern oilfields (Rapdan to Battrum) fran Paleozoic source rocks (Olristopher , 1974, p. 121). The evidence is based on the flow of fornation waters, tectonics , as well as on the absence of source rocks. The following oonclusions are tentatively made:

1. the source rocks are Paleozoic carbonates ;

2. the oil migrated into the Mannville of Saskatchewan by fonnation water flowing fran the deeper part of the basin through fracture conduits in the Paleozoic;

3. the latest phase of this migration coin­c ides with the build up of pressure from the hydraulic head provided by uplift of the central ltlntana region during the La.ramide (Late cretaceous-Miocene) orogeny.

Another observation is that the JX)tentiaretric surface lies at or above bedrock artesia~ l evels

in nost of the province west of the 400 m contour, with the exception of the Tertiary uplands in the southwest. This may account for nruch of the salinit y in the Late Terti ary and Early Quaternary aquifers of the region .

Acknowledgrrents

The author is obligated to D.M. Kent of the Universi ty of Regina, and hi s colleagues , P. Qlliov, J .A. Lorsong , D.F . Paterson and P. Rarnaekers .for the i nterchange of ideas necessary to this =rk.

References

Christopher, ,J.E. (1961) : Transi tional Devonian, Mississippian Formations of Southern Saskatchewan: Saskatchewan Dept. Mineral Resources Rept. No. 61 , 63 p.

(1974): The Upper Jurassic Vanguard and Lo.ver Cretaceous Mannville Groups of Southwestern Saskatchewan : Dept. Mineral Resources Rept. No. 15, 349 p.

(1980) : The ~r Cretaceous Mannville Group of Saskatchewan - A Tectonic overview in Beck, L. S., Christapher, J .E.,rurl Kent , D.M., eds. , Lloydminster and Beyond: Geology of Mannville Hydrocarbon Reservoirs; Saskat­chewan Geological Society Spec . Pub . No. 5, p. 3- 32 .

Evans , C.R., Rogers, M.A. , Bailey , N. J .L. (1971): Evolution and alteration of Petroleum in Western Canada: Chem. Geology, v . 8 , p . 147-170.

Kent, D. M. (1968) : The Geol ogy of the Upper Devonian Saskatchewan Group and Equivalent !bcks in 1-\estern Saskatchewan and Adjacent Areas : Saskatchewan Dept . Mineral Resources Rept. No. 96, 108 p.